Method for making electrostrictive composite

ABSTRACT

A method for making an electrostrictive composite is provided. A number of carbon nanotubes, a number of reinforcing particles, and a polymer precursor are provided. The carbon nanotubes, the reinforcing particles, and the polymer precursor are mixed to obtain a mixture. The polymer precursor in the mixture is polymerized and cured.

RELATED APPLICATIONS

This application is a continuation application of U.S. patent application Ser. No. 12/482,040, filed on Jun. 10, 2009, and entitled, “ELECTROSTRICTIVE COMPOSITE AND METHOD FOR MAKING THE SAME,” which claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 200810067725.7, filed on Jun. 13, 2008 in the China Intellectual Property Office.

BACKGROUND

1. Technical Field

One disclosure relates to a carbon nanotube based electrostrictive composite and method for making the same.

2. Description of Related Art

Electrostrictive composites are materials that can convert electrical energy to mechanical energy, thus imparting a force, while a current or voltage is applied. Electrostrictive composites have been called artificial muscles due to their similar motion properties.

Referring to FIG. 6, a flexible electrothermal composite 10 according to a prior art is shown. The flexible electrothermal composite 10 includes a flexible polymer matrix 14 and a plurality of carbon nanotubes 12 dispersed therein. The carbon nanotubes 12 cooperatively form a conductive network in the flexible polymer matrix 14. The flexible electrothermal composite 10 is made by the following steps of: (a) preparing a solution of a polymer precursor; (b) immersing carbon nanotubes in the solution and ultrasonically cleaning the solution; and (c) polymerizing and curing the polymer precursor.

However, the Young's modulus of the flexible electrothermal composite 10 is relatively small.

What is needed, therefore, is to provide an electrostrictive composite having a relatively larger Young's modulus and method for making the same.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of one electrostrictive composite and method for making the same can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of one electrostrictive composite and method for making the same.

FIG. 1 is a schematic view of an electrostrictive composite.

FIG. 2 is a schematic view of the electrostrictive composite of FIG. 1 before and after expansion.

FIG. 3 is a schematic view of an electrostrictive composite having a sandwich structure.

FIG. 4 is a schematic view of an electrostrictive composite having a multi-layer structure.

FIG. 5 is a flow chart of a method for making the electrostrictive composite of FIG. 1.

FIG. 6 is a schematic view of a flexible electrothermal composite according to the prior art.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one embodiment of one electrostrictive composite and method for making the same, in at least one form, and such exemplifications are not to be construed as limiting the scope of the disclosure in any manner.

DETAILED DESCRIPTION

References will now be made to the drawings to describe, in detail, various embodiments of one electrostrictive composite and method for making the same.

Referring to FIG. 1, an electrostrictive composite 20 according to one embodiment is shown. The electrostrictive composite 20 includes a flexible polymer matrix 22, a plurality of carbon nanotubes 24, and a plurality of reinforcing particles 26. The carbon nanotubes 24 and reinforcing particles 26 are dispersed in the flexible polymer matrix 22. Shape and size of the electrostrictive composite 20 are arbitrary. In one embodiment, the electrostrictive composite 20 is rectangular, and the carbon nanotubes 24 and the reinforcing particles 26 are uniformly dispersed in the flexible polymer matrix 22.

A weight percentage of the flexible polymer matrix 22 in the electrostrictive composite 20 ranges from about 80% to about 98.9%. The flexible polymer matrix 22 includes a material made of silicone elastomer, polyester, polyurethane, epoxy resin, polymethyl methacrylate (PMMA) or combinations thereof. In one embodiment, the flexible polymer matrix 22 is silicone elastomer.

A weight percentage of the sum of the carbon nanotubes 24 and reinforcing particles 26 in the electrostrictive composite 20 is less than 20%. A weight ratio of the carbon nanotubes 24 to the reinforcing particles 26 is greater than or equal to 1:1. A weight percentage of the reinforcing particles 26 in the electrostrictive composite 20 can range from about 1% to about 10% and a weight percentage of the carbon nanotubes 24 in the electrostrictive composite 20 can range from about 0.1% to about 10%. In one embodiment, a weight percentage of the reinforcing particles 26 in the electrostrictive composite 20 ranges from about 1% to about 5% and a weight percentage of the carbon nanotubes 24 in the electrostrictive composite 20 ranges from about 0.1% to about 5%.

The carbon nanotubes 24 can be single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes, or combinations thereof. A length of the carbon nanotubes 24 can be greater than 1 micrometer. The length of the carbon nanotubes 24 can range from about 50 micrometers to about 900 micrometers in one embodiment. The carbon nanotubes 24 are flexible and have excellent electricity to heat conversion efficiency. The carbon nanotubes 24 are in contact with each other to form a conductive network in the flexible polymer matrix 22, thus the electrostrictive composite 20 is conductive. While a voltage is applied to the electrostrictive composite 20, the carbon nanotubes conductive network will rapidly heat and expand the flexible polymer matrix 22.

The reinforcing particles 26 can be made of ceramic, metal, metal oxide, metal nitride, glass or combinations thereof. An effective diameter of the reinforcing particles 26 can range from about 1 nanometer to about 10 micrometers. The reinforcing particles 26 can reduce the thermal response time of the electrostrictive composite 20 due to its high thermal conductivity. The reinforcing particles 26 can enhance the Young's modulus of the electrostrictive composite 20 and raise its propelling power capability during expansion.

Testing was performed on the electrostrictive composite 20 wherein the flexible polymer matrix 22 was silicone elastomer with a weight ratio of 94 wt %, the carbon nanotubes 24 were multi-walled carbon nanotube with a weight ratio of 4 wt %, and the reinforcing particles 26 were boron nitride (BN) particles with a weight ratio of 2 wt %. The Young's modulus of the electrostrictive composite 20 was 1.92 MPa. In another test, the flexible polymer matrix 22 was silicone elastomer with a weight ratio of 94 wt %, the carbon nanotubes 24 were multi-walled carbon nanotube with a weight ratio of 4 wt %, and the reinforcing particles 26 were aluminum oxide (Al₂O₃) particles with a weight ratio of 2 wt %. The Young's modulus of the electrostrictive composite 20 was 1.57 MPa. Another test found that the Young's modulus of a composite consisting of silicone elastomer of 96 wt % and carbon nanotubes of 4 wt % was 1.2 MPa.

The work principle of the electrostrictive composite 20 is described as follows. When a voltage is applied to the electrostrictive composite 20, a current flows through the carbon nanotube conductive network. The electrical energy absorbed by the carbon nanotubes 24 results in local thermal confinement, which breaks the thermodynamic equilibrium therearound. The current and temperature increase simultaneously and rapidly until another thermodynamic equilibrium is achieved. The temperature of the carbon nanotubes 24 rises by absorbing electrical energy, resulting in a temperature increase of the flexible polymer matrix 22 due to the high thermal conductance of the carbon nanotubes. That can lead to an expansion of the electrostrictive composite 20 along its length and width.

The expansion coefficient of the electrostrictive composite 20 ranges from about 0.66×10⁻⁴ K⁻¹ to about 5.28×10⁻⁴ K⁻¹. Referring to FIG. 2, in one embodiment, the expansion coefficient of the electrostrictive composite 20 is tested. In the tested electrostrictive composite 20, the flexible polymer matrix 22 is silicone elastomer of 91wt %, the carbon nanotubes 24 are multi-walled carbon nanotube of 5 wt %, and the reinforcing particles 26 are Al₂O₃ particle of 4 wt %. An effective diameter of the reinforcing particles 26 ranges from about 10 nanometers to about 100 nanometers. The expansion coefficient a of the electrostrictive composite 20 is calculated according to following formula

$\alpha = {{\frac{{L\; 2} - {L\; 1}}{L\; 1} \times \frac{1}{\Delta \; T}} = \frac{ɛ}{\Delta \; T}}$

where L1 is the original length of the electrostrictive composite 20, L2 is the length of the electrostrictive composite 20 after expansion, ΔT is the increase of the temperature of the electrostrictive composite 20, c is the strain. In one embodiment, the L1 is 4 millimeters. The L2 increases to 4.2 millimeters after a voltage of 40 V is supplied for about 2 minutes. The increase of the length ΔL is 0.2 millimeters. The increase of the temperature ΔT is 150K. The strain c of the electrostrictive composite 20 is calculated to be is 5%. The expansion coefficient a of the electrostrictive composite 20 is calculated to be 3.3×10⁻⁴ K⁻¹.

In other embodiments, the electrostrictive composite 20 can further include at least one flexible polymer layer 28 located on at least one surface of the flexible polymer matrix 22. The electrostrictive composite 20 with the flexible polymer layer 28 on at least one surface of the flexible polymer matrix 22 can be an insulator. A thickness ratio of the flexible polymer layer to the flexible polymer matrix 22 can range from about 1% to about 10%. In one embodiment, the flexible polymer matrix 22 can be sandwiched between two silicone elastomer layers serving as the flexible polymer layers 28 as shown in FIG. 3. Alternatively, the electrostrictive composite 20 can be a multi-layer structure as shown in FIG. 4.

Referring to FIG. 6, a method of making the electrostrictive composite 20 includes the following steps of: (a) providing a plurality of carbon nanotubes, a plurality of reinforcing particles and a polymer precursor; (b) mixing the carbon nanotubes, reinforcing particles and the polymer precursor to obtain a mixture; and (c) polymerizing and curing the polymer precursor in the mixture.

In step (a), the carbon nanotubes can be obtained by a conventional method such as chemical vapor deposition (CVD), arc discharging, or laser ablation. In one embodiment, the carbon nanotubes are obtained by the following substeps of: providing a substrate; forming a carbon nanotube array on the substrate by a chemical vapor deposition method; peeling the carbon nanotube array off the substrate by a mechanical method, thereby achieving a plurality of carbon nanotubes. The carbon nanotubes in the carbon nanotube array can be substantially parallel to each other.

The reinforcing particles can be made by a sol-gel or ball milling process. The polymer precursor can be selected according to the flexible polymer. The polymer precursor generally includes a prepolymer or a monomer. The prepolymer can be silicone elastomer prepolymer, polyester prepolymer, polyurethane prepolymer, epoxy resin prepolymer, PMMA prepolymer or combination thereof. In one embodiment, the flexible polymer is silicone elastomer, and the prepolymer is silicone elastomer prepolymer.

In step (b), when the polymer precursor is liquid, the carbon nanotubes and reinforcing particles can be added into the polymer precursor directly to obtain a liquid mixture. When the polymer precursor is solid or glue-like, step (b) can include the substeps of: (b1) dissolving the polymer precursor into a volatilizable solvent to obtain a solution of polymer precursor; and (b2) adding the carbon nanotubes and the reinforcing particles into the solution to obtain a liquid mixture. In one embodiment, the component A of the silicone elastomer is dissolved into ethyl acetate. The component A can be hydroxyl terminated polydimethylsiloxane.

After step (b), an optional step (d) of ultrasonically treating the liquid mixture can be carried out. In step (d), ultrasonically treatment of the liquid mixture is performed in an ultrasonic processor for about 10 minutes to reduce the size of the carbon nanotubes and reinforcing particles. In order to avoid the carbon nanotubes conglomerating with each other in the solution, step (d) further includes the following steps of: ultrasonically agitating the solution for a few minutes to uniformly disperse the carbon nanotubes therein. In one embodiment, ultrasonic agitation is performed in an ultrasonic cleaner for about 3 hours.

When the polymer precursor is dissolved into a volatilizable solvent, an optional step (e) of removing the volatilizable solvent can be performed before the step (c). In one embodiment, the liquid mixture is heated in an oven at a temperature ranging from about 80° C. to about 120° C. until all the ethyl acetate is volatilized.

In step (c), the polymer precursor is polymerized with an initiator to obtain a flexible polymer matrix having carbon nanotubes and reinforcing particles dispersed therein. In one embodiment, the initiator includes a solution of ethanol or deionized water having component B of the silicone elastomer dispersed therein. The initiator is added in the solution of the prepolymer having component A of the silicone elastomer to obtain a mixture solution, in order to polymerize the prepolymer. A weight ratio of the initiator and the prepolymer can be about 6:100. In one embodiment, the component A can be hydroxyl terminated polydimethylsiloxane and the component B can be tetraethoxysilane. Then, after ultrasonically agitating the mixture solution, sediment is collected. The sediment (a glue-like material) having the carbon nanotubes and the reinforcing particles uniformly dispersed therein is the electrostrictive composite.

An optional step (f) of keeping the electrostrictive composite in a vacuum for a period of time to remove bubbles therein can be carried out after step (c). An optional step (g) of pressing the electrostrictive composite can be carried out to obtain a smooth electrostrictive composite after step (f). In one embodiment, the electrostrictive composite is pressed by a smooth presser for about 12 hours to about 18 hours to obtain a planar electrostrictive composite. Furthermore, the planar electrostrictive composite can be cut to have a rectangular shape.

In addition, a step (h) of forming a flexible polymer layer on at least one surface of the flexible polymer matrix can be carried out after step (g). Step (h) can include the substeps of: (h1) providing a liquid prepolymer; (h2) adding initiator into the liquid prepolymer to obtain a liquid mixture; and (h3) immersing the electrostrictive composite into the liquid mixture.

It is also to be understood that the above description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.

Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the disclosure. Variations may be made to the embodiments without departing from the spirit of the disclosure as claimed. The above-described embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure. 

What is claimed is:
 1. A method for making an electrostrictive composite, the method comprising the following steps of: (a) providing a plurality of carbon nanotubes, a plurality of reinforcing particles and a polymer precursor; (b) mixing the carbon nanotubes, the reinforcing particles and the polymer precursor to obtain a mixture; and (c) polymerizing and curing the polymer precursor in the mixture.
 2. The method of claim 1, wherein step (b) comprises the substeps of: (b1) dissolving the polymer precursor into a volatilizable solvent to obtain a solution of polymer precursor; and (b2) adding the carbon nanotubes and the reinforcing particles into the solution of polymer precursor to obtain the mixture.
 3. The method of claim 2, further comprising a step (e) of removing the volatilizable solvent, before step (c).
 4. The method of claim 1, further comprising a step (d) of ultrasonically treating the mixture, after step (b).
 5. The method of claim 1, wherein the polymer precursor is polymerized with an initiator to form the electrostrictive composite.
 6. The method of claim 5, further comprising a step (f) of keeping the electrostrictive composite in a vacuum for a period of time, after step (c).
 7. The method of claim 6, further comprising a step (g) of pressing the electrostrictive composite, after step (f).
 8. The method of claim 7, further comprising a step (h) of forming a flexible polymer layer on one surface of the electrostrictive composite, after step (g).
 9. The method of claim 1, wherein a weight percentage of the carbon nanotubes and reinforcing particles in the electrostrictive composite is less than 20%.
 10. The method of claim 1, wherein a weight ratio of the carbon nanotubes to the reinforcing particles is greater than or equal to 1:1.
 11. The method of claim 1, wherein a weight percentage of the reinforcing particles in the electrostrictive composite ranges from about 1% to about 10%.
 12. The method of claim 1, wherein the reinforcing particles are made of a material selected from the group consisting of ceramic, metal, metal oxide, metal nitride, glass, and combinations thereof.
 13. The method of claim 1, wherein an effective diameter of the reinforcing particles ranges from about 1 nanometer to about 10 micrometers.
 14. The method of claim 1, wherein a weight percentage of the carbon nanotubes in the electrostrictive composite ranges from about 0.1% to about 10%.
 15. The method of claim 1, wherein the carbon nanotubes are selected from the group consisting of single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes, and combinations thereof. 